Our general aim is to characterize the cellular processes affected by two new marine toxins latrunculin A and B and determine their site and molecular mode of action. We have, recently, isolated and purified these toxins from the Red Sea sponge Latrunculia magnifica and determined their structures. Preliminary evaluation of the effects of these toxins on cultured mammalian cells revealed striking changes in cell morphology and microfilament organization. We plan to expand these observations along a number of parallel lines. 1. Using cultured mouse neuroblastoma and fibroblast cells we will further characterize the biological effects of latrunculins by investigating their dose and time dependent action on the morphology of living cells and on microfilament organization as visualized by indirect immunofluorescence microscopy. We will compare the biological effects of the latrunculins with those of the cytochalasins and determine whether the latrunculins affect additional cellular processes. 2. We will test directly the effects of latrunculins on the time course and extent of actin polymerization in vitro utilizing viscosity and spectrophotometric measurements. 3. We will investigate the chemical behavior and conformation of the latrunculin molecules in solution and carry out a structure-activity study of each of the four main functionalities of the molecule to determine the active site. A model compound(i) will be synthesized to study the chemistry of the cyclic hemiketal 2-thiazolidinone bicyclic system. 4. We will label the lantrunculins radioactively and use the labeled compounds to probe the latrunculin binding sites. The long term objectives are: 1. To understand the molecular basis by which the latrunculins alter microfilament structure. 2. To determine the essential functionalities for the biological activities of the latrunculins. 3. To use the latrunculins as a cytological probe of microfilament function and gain insight into the role of microfilaments in the regulation of such key cellular processes as motility, shape determination, nerve cell differentiation, topographical organization of specialized surface membrane components and cell transformation.